This invention relates generally to mass transfer columns and, more particularly, to an apparatus and method for supporting and sealing trays within such columns.
Mass transfer columns, including heat exchange columns, typically include an upright shell and a plurality of horizontally disposed trays that are positioned within the shell and are used to facilitate mass or heat transfer between fluid streams flowing within the column. The fluid streams are normally one or more downwardly flowing liquid streams and one or more ascending vapor streams, although other combinations of fluid streams are possible. Each tray includes a plurality of vapor passages that allow the vapor stream to ascend through the tray for interaction with the liquid stream flowing across an upper surface of the tray. A plurality of such trays are normally supported in vertically spaced relationship by support rings that are welded to the inner surface of the shell and underlie the outer peripheral edge portion of the trays.
During revamping of mass transfer columns, it is often necessary to change the vertical spacing between successive trays. Considerable time and effort, however, is normally required to remove the existing support rings and weld new support rings at the desired location on the column shell. In addition, the process of cutting out the old support rings and welding new support rings may be hazardous if combustible fluids or residues are still present in the column or if the structural integrity of the column shell is weakened or otherwise negatively affected by the cutting and welding. Post-revamp stress relieving and pressure testing of the column may be necessary to ensure that the column remains safe for use after replacement of the support ring. This post-revamp stress relieving and pressure testing of the column further adds to the costs and delays associated with replacing the support rings in revamped columns.
In order to reduce the number of support rings that must be welded to the column shell in revamped as well as new columns, it is known to use support rings that are welded to the column shell to support one or more vertically adjacent support rings that are not welded to the column shell. Vertical rods known as pedestals extend upwardly from the welded support rings to position and support the overlying support rings that are not welded to the column shell. The support rings that are supported in this manner are known as expansion or floating or surrogate support rings and their use can greatly reduce the time and effort required to install the support rings within the column.
One problem resulting from the use of these floating or surrogate support rings is the difficulty in obtaining a fluid-tight seal between the support ring and the column shell. While some leakage between the support ring and column shell is acceptable, an out-of-round column shell or support ring may cause an unacceptable level of fluid leakage that materially affects the performance of the column. One approach to minimizing this gap has been to use a floating or surrogate support ring that can be expanded in circumference. These expandable support rings are commonly known as expansion rings and are constructed as one or more incomplete ring segments. A 90° flange extends radially from one end of the ring segment and an overlapping 90° flange extends downwardly from a top edge of the adjacent end of the ring segment. The overlapping 90° flange has a horizontal segment and a downwardly extending segment. The horizontal segment is of a preselected length to space the downwardly extending segment a preselected distance from the end of the associated ring segment. A bolt extends through the overlapping portions of the two 90° flanges and, when a nut is tightened on the bolt against an opposite face of one of the flanges, the flanges are brought closer to each other to cause an expansion in the circumference of the ring segment.
One problem associated with the design of the expansion ring described above is the flanges may deform from their 90° orientation as the nut is tightened to cause expansion of the rings. This deformation of the flanges limits the amount that the ring can be expanded to seal against the inner surface of the column shell. While the flanges can be formed of thicker gauge material to resist such deformation, the use of lighter gauge material is generally preferred because it is easier to carry into the column during installation. The use of thicker material for the flanges is also problematic because a source for such material may not be readily available. When the expansion ring described above is formed from two or more ring segments, each segment has only one correct orientation and the installer must pay careful attention to the order in which the segments are installed to ensure that the expansion ring is assembled correctly.
A need has thus arisen for an expansion ring that can be expanded to form a tighter seal against the column shell than can normally be achieved with conventional expansion rings. A need has also arisen for an expansion ring that can offers a wide range of movement from the initial installation to the fully expanded position without allowing large quantities of fluid to flow through the gap between the ends of the ring or ring segments. A need has also arisen for an expansion ring having multiple ring segments that can be assembled together in more than one arrangement to facilitate assembly of the expansion ring.